Spike Train Statistics from Empirical Facts to Theory: The Case of the Retina - Modeling in Computational Biology and Biomedicine - page 253

a b

Retinal
pigment
epithelium

Light

Pigment
epithelial cell

Rod
Cone
Outer limiting
membrane

Rods and
cones layer
Outer
nuclear layer
Outer
plexiform layer
Inner
nuclear layer
Inner
plexiform layer
Ganglion
cell layer
Nerve fibre
layer

P

H

Müller cell

Horizontal cell

B

Blood vessel
Amacrine cell

A

Ganglion cell

G

Inner limiting
membrane Optic nerve

Fig. 8.1 Processing steps of the visual stream. ( a ) The cellular organization of the retina
(From Expert Reviews in Molecular Medicine by Cambridge University Press 2004); ( b )Main
connectivity structure between retina cells types (Reproduced from [ 24 ]). Chemical synapses
are represented by circles , either excitatory ( closed circles ) or inhibitory ( open circles ). Electric
synapses ( gap junctions ) are marked by resistor symbols

layers [ 37 , 77 ](Fig. 8.1 a). The outer nuclear layer (ONL) contains the rods and
cones photoreceptors (P) somata; the inner nuclear layer (INL) contains bipolar
(B), horizontal (H) and amacrine cells (A). Finally, the most internal nuclear layer is
composed with ganglion cells (G) and displaced amacrine cells. The outer plexiform
layer (OPL) corresponds to synaptic contacts between P, B and H cells. The inner
plexiform layer (IPL) corresponds to synaptic contacts between B, A and G cells.
The retina is about 300-500 m thick, depending on species, and has about 100-

130 millions of photoreceptors, 10-12 millions of bipolar, horizontal and amacrines
cells and 0.4-1.6 millions of G cells. Together with this high and compact number
of cells there is a very large number of synapses present in dendrites and axons
terminal, that has been roughly estimated to one billion of synapses [ 58 ]. The
retina is also rich in terms of the variability of neurotransmitters, where rods,
cones, and bipolar cells liberate glutamate, horizontal and amacrines cells can
liberate gaba, glycine, serotonin, acetylcholine, dopamine among others. Together
with the richness in chemical slow synapses circuits the retina has a variety
of electrical (“gap-junctions”), fast synapses endowing the retina with specific
functional circuits.
Single photons are converted by photoreceptors into a graded change in the
resting potential, resulting in a neurotransmitter liberation (glutamate) into the
synaptic region connecting photoreceptors with B and H cells. Those cells make
synapses with G and A cells. Therefore, photons fluxes generate a chain of changes
in the resting potential of B,H,A, and G cells with consequence the emission of
action potentials (“spikes”) by G cells. They are the principal neural encoders
through the integration of neural signals. The retina output, formed by spike train
sequence, is carried by different types of G cells through the optical nerve to the
brain higher visual structures: e.g., lateral geniculate nucleus (LGN) or visual cortex
layers (Fig. 8.2 ).

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